U.S. patent application number 10/503042 was filed with the patent office on 2005-10-27 for method for producing a semiconductor element.
Invention is credited to Fehrer, Michael, Hahn, Berthold, Harle, Volker, Kaiser, Stephan, Otte, Frank, Plossl, Andreas.
Application Number | 20050239270 10/503042 |
Document ID | / |
Family ID | 27664551 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050239270 |
Kind Code |
A1 |
Fehrer, Michael ; et
al. |
October 27, 2005 |
Method for producing a semiconductor element
Abstract
A method for producing a semiconductor component, in particular
a thin-film component, a semiconductor layer being separated from a
substrate by irradiation with a laser beam having a plateaulike
spatial beam profile. Furthermore, the semiconductor layer, prior
to separation, is applied to a carrier with an adapted thermal
expansion coefficient. The method is suitable in particular for
semiconductor layers containing a nitride compound
semiconductor.
Inventors: |
Fehrer, Michael; (Bad
Abbach, DE) ; Hahn, Berthold; (Hemau, DE) ;
Harle, Volker; (Laaber, DE) ; Kaiser, Stephan;
(Regensburg, DE) ; Otte, Frank; (Hannover, DE)
; Plossl, Andreas; (Regensburg, DE) |
Correspondence
Address: |
COHEN, PONTANI, LIEBERMAN & PAVANE
551 FIFTH AVENUE
SUITE 1210
NEW YORK
NY
10176
US
|
Family ID: |
27664551 |
Appl. No.: |
10/503042 |
Filed: |
May 14, 2005 |
PCT Filed: |
January 30, 2003 |
PCT NO: |
PCT/DE03/00260 |
Current U.S.
Class: |
438/463 ;
257/E21.122; 257/E21.567 |
Current CPC
Class: |
H01L 2924/01079
20130101; H01L 2924/01057 20130101; H01L 2924/10329 20130101; H01L
2924/12036 20130101; C30B 29/403 20130101; H01L 24/83 20130101;
H01L 2224/29111 20130101; H01L 2924/01006 20130101; B23K 2103/172
20180801; H01L 2924/00013 20130101; H01L 2924/01068 20130101; H01L
2224/29101 20130101; H01L 2224/83001 20130101; H01L 2924/01029
20130101; H01L 2924/12041 20130101; H01L 2924/157 20130101; H01L
2924/0106 20130101; H01L 24/29 20130101; B23K 26/50 20151001; H01L
2924/01074 20130101; H01L 2924/01082 20130101; H01L 2924/12042
20130101; B23K 26/40 20130101; H01L 21/76251 20130101; H01L
2224/8319 20130101; H01L 2924/01042 20130101; H01L 21/2007
20130101; H01L 2924/014 20130101; H01L 2224/29 20130101; H01L
2924/0105 20130101; H01L 2924/01019 20130101; H01L 2924/15763
20130101; H01L 2924/01013 20130101; H01L 2924/01033 20130101; H01L
33/0093 20200501; H01L 2924/01078 20130101; H01L 2924/01005
20130101; C30B 29/40 20130101; H01L 2924/01049 20130101; C30B 33/00
20130101; H01L 2224/29298 20130101; H01L 2924/0132 20130101; C30B
29/406 20130101; H01L 2924/01032 20130101; H01L 2924/0133 20130101;
H01L 2224/29144 20130101; H01L 2924/01023 20130101; H01L 2224/29144
20130101; H01L 2924/0105 20130101; H01L 2224/29101 20130101; H01L
2924/014 20130101; H01L 2924/00 20130101; H01L 2924/0133 20130101;
H01L 2924/01026 20130101; H01L 2924/01027 20130101; H01L 2924/01028
20130101; H01L 2924/0132 20130101; H01L 2924/01046 20130101; H01L
2924/01049 20130101; H01L 2924/0132 20130101; H01L 2924/0105
20130101; H01L 2924/01079 20130101; H01L 2924/3512 20130101; H01L
2924/00 20130101; H01L 2924/00014 20130101; H01L 2224/29111
20130101; H01L 2924/01079 20130101; H01L 2924/00014 20130101; H01L
2924/15763 20130101; H01L 2924/01042 20130101; H01L 2924/00013
20130101; H01L 2224/29099 20130101; H01L 2924/00013 20130101; H01L
2224/29199 20130101; H01L 2924/00013 20130101; H01L 2224/29299
20130101; H01L 2924/00013 20130101; H01L 2224/2929 20130101; H01L
2924/12036 20130101; H01L 2924/00 20130101; H01L 2924/12042
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
438/463 |
International
Class: |
H01L 021/301 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2002 |
DE |
102 03 795.7 |
Sep 20, 2002 |
DE |
102 43 757.2 |
Claims
1. A method for producing a semiconductor component, in which a
semiconductor layer (2) is separated from a substrate (1) by
irradiation with a laser beam (6), wherein the laser beam (6) has a
plateaulike spatial beam profile (7).
2. A method for producing a semiconductor component, in which a
semiconductor layer (2) is separated from a substrate (1) by
irradiation with a laser beam (6), wherein the laser beam (6) is
generated by an excimer laser.
3. The method as claimed in claim 1, wherein the excimer laser
contains a noble gas-halogen compound, in particular XeF, XeBr,
XeCl, KrCl or KrF, as laser-active medium.
4. The method as claimed in claim 2, wherein the laser beam (6) has
a plateaulike spatial beam profile (7).
5. (The method as claimed in claim 1, wherein the laser beam (6)
has a rectangular or trapezoidal spatial beam profile.
6. The method as claimed in claim 1, wherein the laser beam (6) is
generated by a laser in pulsed operation.
7. The method as claimed in claim 1, wherein the wavelength of the
laser beam (6) lies between 200 nm and 400 nm.
8. The method as claimed in claim 1, wherein the laser beam (6) is
focused onto the semiconductor layer (2) in such a way that, within
the irradiated region, the energy density generated by the laser
beam (6) lies between 100 mJ/cm.sup.2 and 1000 mJ/cm.sup.2, in
particular between 150 mJ/cm.sup.2 and 800 mJ/cm.sup.2.
9. The method as claimed in claim 1, wherein a plurality of
individual regions (8) of the semiconductor layer (2) are
irradiated successively.
10. The method as claimed in claim 9, wherein the individual
regions (8) are arranged in area-filling fashion such that a
spatially approximately constant intensity distribution (10)
results, in a manner integrated with respect to time, for a
predominant part of the irradiated semiconductor layer (2).
11. The method as claimed in claim 1, wherein the laser beam (6)
has, at the location of the semiconductor layer (2), a beam area
with a longitudinal dimension (a) and a transverse dimension (b),
the longitudinal dimension (a) being greater than the transverse
dimension (b) and the semiconductor layer (2) is moved relative to
the laser beam (6) during the irradiation along the direction of
the transverse dimension (b).
12. The method as claimed in claim 1, wherein the substrate (1) is
at least partly transmissive to the laser beam (6) and the
semiconductor layer (2) is irradiated through the substrate
(1).
13. A method for producing a semiconductor component, in which a
semiconductor layer (2) is separated from a substrate (1) by
irradiation with a laser beam (6), wherein prior to separation from
the substrate (1), the semiconductor layer (2) is applied,
preferably soldered, onto a carrier (4) by the side remote from the
substrate (1).
14. The method as claimed in claim 13, wherein the laser beam (6)
is pulsed.
15. The method as claimed in claim 13, wherein the thermal
expansion coefficient of the carrier a.sub.T is chosen in a manner
coordinated with the beam profile and/or the pulse length of the
laser beam pulses and with the thermal expansion coefficient of the
semiconductor layer a.sub.HL and the thermal expansion coefficient
a.sub.S of the substrate, in order to reduce strains between
substrate, semiconductor layer and carrier during production.
16. The method as claimed in claim 15, wherein the thermal
expansion coefficient of the carrier a.sub.T is chosen to be nearer
to the thermal expansion coefficient of the semiconductor layer
a.sub.HL than to the thermal expansion coefficient a.sub.S of the
substrate.
17. The method as claimed in claim 15, wherein the thermal
expansion coefficient of the carrier a.sub.T differs from the
thermal expansion coefficient a.sub.S of the substrate by 45% or
less, preferably by 40% or less.
18. The method as claimed in claim 15, wherein the thermal
expansion coefficient of the carrier a.sub.T differs from the
thermal expansion coefficient a.sub.HL of the semiconductor layer
by 35% or less, preferably by 25% or less.
19. The method as claimed in claim 15, wherein the carrier has a
thermal expansion coefficient of between approximately
4.3*10.sup.-6K.sup.-1 and approximately 5.9*10.sup.-6K.sup.-1,
preferably between approximately 4.6*10.sup.-6K.sup.-1 and
approximately 5.3*10.sup.-6K.sup.-1.
20. The method as claimed in claim 13, wherein the carrier (4)
contains gallium arsenide, silicon, copper, iron, nickel and/or
cobalt.
21. The method as claimed in claim 13, wherein the carrier contains
molybdenum.
22. The method as claimed in claim 13, wherein the carrier contains
an iron-nickel-cobalt alloy.
23. The method as claimed in claim 13, wherein the carrier contains
tungsten.
24. The method as claimed in claim 13, wherein the carrier contains
germanium.
25. The method as claimed in claim 14, wherein a large pulse length
of the laser beam pulses, in particular a pulse length of greater
than 15 ns, is chosen for the separation of the semiconductor layer
from the substrate.
26. The method as claimed in claim 15, wherein the thermal
expansion coefficient of the carrier a.sub.T differs from the
thermal expansion coefficient a.sub.HL of the semiconductor layer
by 35% or more, and in which a small pulse length of the laser beam
pulses, in particular a pulse length of less than approximately 15
ns, is chosen for the separation of the semiconductor layer from
the substrate.
27. The method as claimed in claim 13, wherein the semiconductor
layer (2) is soldered onto the carrier (4) by means of a solder
containing gold and/or tin or palladium and/or indium.
28. The method as claimed in claim 13, wherein before the
semiconductor layer (2) is connected to the carrier (4), a
metallization is applied to that side of the semiconductor layer
(2) which is remote from the substrate (1).
29. The method as claimed in claim 28, wherein the metallization
contains gold and/or platinum.
30. The method as claimed in claim 1, wherein the semiconductor
layer (2) comprises a plurality of individual layers.
31. The method as claimed in claim 1, wherein the semiconductor
layer (2) or at least one of the individual layers contains a
nitride compound semiconductor.
32. The method as claimed in claim 31, wherein the nitride compound
semiconductor is a nitride compound of elements of the third and/or
fifth main group.
33. The method as claimed in claim 31, wherein the semiconductor
layer (2) or at least one of the individual layers contains
In.sub.xAl.sub.yGa.sub.- 1-x-yN where 0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1 and x+y.ltoreq.1, in particular GaN, AlGaN,
InGaN, AlInGaN, AlN or InN.
34. The method as claimed in claim 1, wherein the substrate (1)
contains silicon, silicon carbide or aluminum oxide, in particular
sapphire.
35. The method as claimed in claim 13, wherein the semiconductor
layer (2) is separated from the substrate (1) by means of a method
as claimed in one of claims 1 to 12.
36. The method as claimed in claim 1, wherein the semiconductor
layer (2) is applied to the substrate (1) by means of an epitaxy
method.
37. The method as claimed in claim 1, wherein the semiconductor
layer (2) has a thickness which is less than or equal to 50
.mu.m.
38. The method as claimed in claim 1, wherein the semiconductor
component is a thin-film component in the case of which the
substrate is at least partly removed from the grown-on
semiconductor layer after the latter has been grown on.
39. The method as claimed in claim 1, wherein the semiconductor
component is an optoelectronic component, in particular a
radiation-generating component such as a luminescence diode.
Description
[0001] The invention relates to a method for producing a
semiconductor component, in which a semiconductor layer is
separated from a substrate by irradiation with a laser beam.
[0002] A method of this type is used for example in the production
of substrateless luminescence diodes based on GaN. Such components
contain a semiconductor body and a carrier part, on which the
semiconductor body is fixed. In order to produce the semiconductor
body, firstly a semiconductor layer is fabricated on a suitable
substrate, subsequently connected to a carrier and then stripped
away from the substrate. Dividing up, for example sawing up, the
carrier with the semiconductor layer arranged thereon produces a
plurality of semiconductor bodies which are in each case fixed on
the corresponding carrier part. What is essential in this case is
that the substrate used for producing the semiconductor layer is
removed from the semiconductor layer and does not simultaneously
serve as a carrier or carrier part in the component.
[0003] This production method has the advantage that different
materials are used for the substrate and the carrier. The
respective materials can thus be adapted, largely independently of
one another, to the various requirements for the production of the
semiconductor layer, on the one hand, and the operating conditions,
on the other hand. Thus, the carrier can be chosen in accordance
with its mechanical, thermal and optical properties independently
of the requirements made of the substrate for the fabrication of
the semiconductor layer.
[0004] The epitaxial production of a semiconductor layer, in
particular, makes numerous special requirements of the epitaxial
substrate. By way of example, the lattice constants of the
substrate and of the semiconductor layer to be applied have to be
adapted to one another. Furthermore, the substrate should withstand
the epitaxy conditions, in particular temperatures of up to in
excess of 1000.degree. C., and be suitable for the epitaxial
accretion and growth of an as far as possible homogeneous layer of
the relevant semiconductor material.
[0005] By contrast, other properties of the carrier such as, by way
of example, electrical and thermal conductivity and also radiation
transmissivity in the case of opto-electronic components come to
the fore for the further processing of the semiconductor body and
operation. Therefore, the materials suitable for an epitaxial
substrate are often only suitable to a limited extent as carrier
part in the component. Finally, it is desirable, particularly in
the case of comparatively expensive epitaxial substrates such as
silicon carbide substrates, for example, to be able to use the
substrates repeatedly.
[0006] The stripping-away of the semiconductor layer from the
substrate is essential for the aforementioned production method.
Said stripping-away can be achieved by irradiating the
semiconductor-substrate interface with laser radiation. In this
case, the laser radiation is absorbed in the vicinity of the
interface, where it effects decomposition of the semiconductor
material.
[0007] The semiconductor layer may be separated from the substrate
for example by means of laser stripping, as described in the
document WO 98/14986. In this case, the frequency-tripled radiation
of a Q-switch Nd:YAG laser at 355 nm is used for stripping GaN and
GaInN layers from a sapphire substrate. The sapphire substrate is
transparent to radiation at this wavelength. The radiation energy
is absorbed in a boundary layer having a thickness of approximately
100 nm at the junction between the sapphire substrate and the GaN
semiconductor layer. At pulse energies of above 200 mJ/cm.sup.2,
temperatures of more than 850.degree. C. are reached at the
interface. The GaN boundary layer decomposes at this temperature to
liberate nitrogen, and the bond between the semiconductor layer and
the substrate is separated.
[0008] In the case of a method of this type, there is the risk of
substrate residues adhering on the semiconductor layer on account
of incomplete material decomposition during the stripping away of
the semiconductor layer. By way of example, microscopic sapphire
grains, so-called "flakes", are often found on a GaN layer
separated from a sapphire substrate in this way.
[0009] The diameter of these sapphire residues typically lies
between 5 .mu.m and 100 .mu.m. The sapphire residues make further
processing of the semiconductor layer more difficult and require a
comparatively high effort to remove them on account of the high
mechanical and chemical resistance of sapphire. This may have the
effect that only parts of the stripped-away semiconductor layer can
continue to be used or the entire layer even becomes unusable.
[0010] Generally, a mechanical stabilization of the semiconductor
layer to be stripped away is necessary since the layer thickness is
so small that otherwise there is the risk of damage, in particular
a break or crack in the layer. By way of example, a connection of
the semiconductor layer, which may also already be partly
processed, to the carrier by means of a material joint is suitable
for the purpose of mechanical stabilization. Such a connection
should be thermostable at least to an extent such that it
withstands without damage the temperatures that occur during
subsequent fabrication steps. Furthermore, said connection should
also remain stable in the event of alternating temperature loads
which may occur, in particular, during operation of the
component.
[0011] Adhesives are often used for fixing the semiconductor layer
on the carrier. In the case of relatively high electrical powers,
problems may result in this case on account of the limited thermal
and electrical conductivity of adhesives. The limited thermal
endurance of such adhesive connections additionally limits the
permissible temperature range of a corresponding component and
consequently the maximum possible power loss.
[0012] It is an object of the present invention to develop an
improved method for producing a semiconductor component in which a
semiconductor layer is separated from a substrate by means of laser
irradiation.
[0013] This object is achieved by means of a method according to
any of patent claims 1, 2 and 13. The dependent claims relate to
advantageous developments of the invention.
[0014] The invention provides for a semiconductor layer to be
separated from a substrate by irradiation with a laser beam, the
laser beam having a plateaulike, in particular rectangular or
trapezoidal, beam profile. This beam profile significantly reduces
the number of substrate residues on the semiconductor layer in
comparison with a conventional separation method.
[0015] A plateaulike beam profile is to be understood to be a
transversal intensity distribution of the laser beam which has a
central region with an essentially constant intensity distribution
adjoined in each case by a flank with falling intensity.
Preferably, the relative fluctuation of the beam intensity in the
central region is less than 5 percent.
[0016] In order to further improve the beam profile, a beam
homogenizer may be arranged downstream of the laser. Furthermore,
it is expedient to use a suitable optical arrangement, for example
a lens system which may comprise correction lenses, attenuators,
mirrors, mask structures and/or projection lenses, for imaging the
laser beam onto the semiconductor layer. In this way, it is
possible to set the energy density required for material
decomposition without impairing the advantageous beam profile.
[0017] By contrast, the lasers used in conventional laser
separation methods generally have a Gaussian beam profile. This
leads to a spatially comparatively greatly varying, inhomogeneous
field distribution on the semiconductor-substrate interface and
consequently to varying degrees of material decomposition. During
the subsequent stripping-away of the semiconductor layer, there is
the risk of substrate residues adhering on the semiconductor layer
at locations of weak or incomplete material decomposition.
[0018] Preferably, in the case of the invention, the laser beam is
generated by an excimer laser. Excimer lasers generally have a
plateaulike, often trapezoidal or rectangular, beam profile.
Furthermore, particularly in the case of excimer lasers with a
noble gas-halogen compound as laser medium, the emission wavelength
lies in the ultraviolet spectral range, which is particularly
suitable for stripping away nitride compound semiconductors.
Moreover, the pulse peak power in the case of excimer lasers, which
typically lies between 1 kW and 100 MW, is so high that even in the
event of mask imagings of the laser beam and after passage through
a plurality of lenses, the energy density suffices for material
decomposition.
[0019] In order to achieve the beam intensity required for the
material decomposition, a pulsed operation is expedient for the
laser. In comparison with a laser in continuous wave operation,
this also reduces the risk of overheating of the semiconductor
layer that is to be stripped away. In the case of a pulsed laser,
the transporting-away of the heat arising as a result of the laser
irradiation can be set optimally through a suitable choice of pulse
duration and pulse spacing.
[0020] In the case of semiconductor layers with a relatively large
lateral extent, it is advantageous for individual regions of the
semiconductor layer that are arranged next to one another to be
irradiated successively, in order to avoid an excessively large
expansion of the beam area. Since, for a given beam power or energy
of the laser pulse, the intensity decreases as the beam area
increases, it is possible, in the event of excessively great beam
expansion, for the decomposition threshold, i.e. the energy density
required for the material decomposition, to be undershot and
complete stripping-away of the semiconductor layer to be
impaired.
[0021] In this case, it is particularly advantageous to guide the
laser beam and/or the substrate with the semiconductor layer
situated thereon such that the irradiated individual regions
produce an area-filling overall arrangement. An approximately
constant spatial intensity distribution for the predominant part of
the irradiated area corresponds to this in a manner integrated with
respect to time, i.e. over the irradiation time period. On account
of this approximately constant intensity distribution, the
stripped-away semiconductor layer has an advantageously small
number of substrate residues or is even free of residues. A
plateaulike, in particular rectangular, spatial beam profile is
particularly advantageous for the abovementioned area-filling
overall arrangement of the irradiated individual regions.
[0022] In a preferred development of the invention, the laser beam
has, at the location of the semiconductor layer or the
semiconductor-substrate interface, a beam area with a longitudinal
dimension and a transverse dimension, the longitudinal dimension
being significantly greater than the transverse dimension.
Preferably, the longitudinal dimension exceeds the transverse
dimension by a factor of 5 to 10, thus producing a linear or
striplike beam area.
[0023] The semiconductor layer is moved during irradiation in a
parallel direction with respect to the transverse dimension, so
that, during irradiation, the linear or striplike beam area sweeps
over the entire semiconductor layer to be stripped away. In this
case, too, an advantageously constant intensity distribution of the
irradiated semiconductor layer results in a manner integrated over
the irradiation time period, a further advantage consisting in the
fact that a simple linear movement of the semiconductor layer
relative to the laser beam suffices. It goes without saying that
this involves a relative movement between semiconductor layer and
laser beam which can be realized both by means of a movement of the
semiconductor layer with a stationary laser beam and by means of a
corresponding guidance of the laser beam with a stationary
semiconductor layer.
[0024] In the case of the invention, it is advantageous to
irradiate the direct interface region between semiconductor layer
and substrate with the laser radiation, so that the radiation
energy is absorbed near the interface and leads to a material
decomposition there. This may be achieved by virtue of the fact
that the substrate is transmissive to the laser radiation and the
semiconductor layer is irradiated through the substrate. In the
case of this arrangement, the absorption of the laser radiation is
generally significantly greater in the semiconductor layer than in
the substrate, so that the laser beam penetrates through the
substrate virtually without any losses and is absorbed on account
of the high absorption near the interface in the semiconductor
layer.
[0025] It should be noted that the radiation absorption need not
necessarily be effected at the location of material decomposition.
The material decomposition may also be effected by the radiation
firstly being absorbed at a different location and then the
absorbed radiation energy being transported to the location of
material decomposition. If appropriate, the radiation could also be
absorbed in the substrate and the radiation energy could
subsequently be transported to the semiconductor layer.
[0026] In a further aspect of the invention, it is provided that,
in order to produce a semiconductor component, a semiconductor
layer is separated from a substrate by means of a laser beam, prior
to separation the semiconductor layer being applied, preferably
soldered, onto a carrier by the side remote from the substrate. A
soldered connection is distinguished by a high thermal and
electrical conductivity in comparison with conventional adhesive
connections.
[0027] The separation itself is preferably effected according to
one of the methods already described. It goes without saying that
although a soldered connection is advantageous in the case of the
separation methods described previously, an adhesive-bonding
connection between carrier and semiconductor layer also lies within
the scope of the invention.
[0028] The solder used is preferably a gold-containing solder, for
example a gold-tin solder. Gold-tin solders having a high
proportion of gold, for example between 65% by weight and 85% by
weight, are particularly preferred in this case.
[0029] The melting point of such a solder is typically 278.degree.
C. and is thus greater than the temperature which usually arises
during the soldering of an electrical component. Thus, by way of
example, the soldering temperature in the course of soldering onto
a printed circuit board is generally less than 260.degree. C. This
prevents the semiconductor body from being stripped away from the
carrier part when the component is soldered in.
[0030] Furthermore, an example of a suitable solder is a
palladium-indium solder, the constituents of which are intermixed
at a comparatively low initial temperature of approximately
200.degree. C., and which has an advantageously high melting point
of in excess of 660.degree. C. after intermixing.
[0031] Such a connection may be produced for example by applying an
indium layer on the semiconductor layer and a palladium layer on
the carrier and then joining together the carrier and the
semiconductor layer under increased pressure at a temperature of
approximately 200.degree. C. or more.
[0032] It goes without saying that it is also possible for the
palladium layer to be applied on the semiconductor layer and the
indium layer to be applied on the carrier. Moreover, it is
advantageous to provide further layers between the semiconductor
layer and the metal layer, said further layers ensuring for example
protection of the semiconductor layer or good adhesion. A layer
sequence with a titanium layer on the semiconductor surface, then a
palladium layer and an indium layer thereon is particularly
advantageous in conjunction with a palladium layer on the
carrier.
[0033] With regard to a low contact resistance and advantageous
soldering properties, it is expedient to provide the semiconductor
layer with a contact metallization on the side facing the carrier,
prior to soldering onto the carrier. A platinum-gold metallization,
for example, is suitable for this purpose.
[0034] In a further aspect of the invention, it is provided that
the thermal expansion coefficient of the carrier a.sub.T is chosen
in a manner coordinated with the thermal expansion coefficient of
the semiconductor layer a.sub.HL and/or the thermal expansion
coefficient of the substrate a.sub.S and also, if appropriate, the
beam profile and the pulse length of the laser beam pulses.
Generally, a coordination of the thermal expansion coefficients is
to be understood to mean that their difference is so small that, in
the temperature range that occurs during production or is provided
in operation, no damage is produced at the semiconductor layer and
the carrier. In particular, this makes it possible to significantly
reduce strains between substrate, semiconductor layer and carrier
during production. The risk of cracking in the carrier and in the
semiconductor layer is thus greatly decreased.
[0035] It has been observed in this respect in the context of the
invention that the spot profiles (intensity profiles) of the laser
pulses used for stripping away the semiconductor layers are often
revealed after the laser bombardment on the semiconductor surface.
In the case where GaN semiconductor layers are stripped away,
metallic gallium remains on the surface, for instance, after the
dissociation of the GaN. The inventors' examinations furthermore
revealed that cracks arise in the GaN material at the edges of the
laser spots, which cracks, during further processing of the
material, lead to the local flaking-away of the semiconductor layer
from the underlying carrier.
[0036] It has now been found that primarily thermal effects are
responsible for this. In order, in the case of a GaN semiconductor
layer, for instance, to achieve a dissociation of the GaN, it is
necessary locally to achieve temperatures of approximately
800.degree. C. to 1000.degree. C. in the semiconductor layer. If
the energy density decreases greatly at the edge of the laser spot,
then the temperatures required for stripping away can be reached
within the laser spot, while the semiconductor material remains
comparatively cold in the direct vicinity of the laser spot.
[0037] Although the temperatures reached at the GaN surface
decrease significantly over the layer thickness of the
semiconductor layer, temperatures of up to 400.degree. C. are still
reached at the carrier side of the semiconductor layer in the
region of the laser spot. Consequently, tensile strains arise on
account of the locally different temperatures in the laser spot and
outside the spot both in the semiconductor layer and in the carrier
on account of the generally different thermal expansion
coefficients of the semiconductor material and the carrier
material, and may lead to the observed formation of cracks in the
semiconductor material at the laser spot edges.
[0038] During the further processing of such semiconductor layers
provided with cracks, the problem arises, for example, that acid
can creep along the cracks under the semiconductor layer and, for
instance, destroys a bonding metallization there.
[0039] In the case of the invention, use is preferably made of
special carrier materials that are adapted in terms of their
thermal properties. In this case, two process steps, in particular,
namely the bonding process and the laser irradiation are taken into
account for the choice of the thermal expansion coefficient of the
carrier a.sub.T.
[0040] During the bonding process, the substrate with the
semiconductor layer epitaxially coated thereon, together with the
carrier, is heated in whole-area fashion to a temperature of
typically approximately 400.degree. C. and subsequently cooled down
again gradually to room temperature. In this step, the strain
balance of the layer assembly substrate/semiconductor layer/carrier
is essentially determined by the substrate and the carrier. If the
thermal expansion coefficients of substrate and carrier, a.sub.S
and a.sub.T, deviate too greatly from one another, then the layer
assembly may buckle as it cools down. Cracks may also form in the
carrier, so that the resulting chip no longer has sufficient
stability.
[0041] This problem is illustrated by way of example in FIG. 7. In
the case of the layer assembly 20 illustrated diagrammatically
there, a GaN semiconductor layer 21 is grown on a sapphire
substrate 22. That side of the semiconductor layer 21 which is
remote from the substrate 22 is provided with a contact
metallization 23. A bonding wafer is soldered as carrier 24 on the
contact metallization 23 at a temperature of approximately
400.degree. C.
[0042] If the thermal expansion coefficient a.sub.T of the carrier
is significantly less than the thermal expansion coefficient
a.sub.S of the sapphire substrate, then cracks 25 may form in the
carrier 24 during this bonding step.
[0043] During the laser irradiation, the semiconductor material is
locally heated within the laser spot to a temperature above the
decomposition temperature of the semiconductor material, while the
substrate material remains cold on account of its substantially
lower absorption of the laser radiation. Since the laser
irradiation eliminates the bond between the semiconductor material
and the substrate by dissociation, the difference between the
thermal expansion coefficients of semiconductor layer and carrier,
a.sub.HL and a.sub.T, determines the strain balance in the layer
assembly. In the event of a large difference between a.sub.HL and
a.sub.T, tensile strains may arise, which may lead to cracking in
the semiconductor material at the locations of the spot edges.
[0044] FIG. 8 elucidates the problem area once again for stripping
away a GaN layer 21 from a sapphire substrate 22. When the layer
assembly 20 is irradiated with short laser pulses 26 of an excimer
laser, the laser radiation is absorbed in a region 27 of the GaN
layer 21 near the boundary and generates temperatures of
800.degree. C. to 1000.degree. C. there. Temperatures of up to
approximately 400.degree. C. are still reached on that side of the
semiconductor layer 21 which is remote from the substrate and in
the adjoining region 28. The GaN layer 21 and the contact
metallization 23 remain comparatively cold outside the laser spot.
The temperature in the regions 29 and 30 which laterally directly
adjoin the laser spot is typically significantly less than
300.degree. C. In the event of greatly different thermal expansion
coefficients between the GaN layer 21 and the material of the
carrier 24 or the bonding wafer, cracks 31 may thus arise in the
epitaxial GaN layer 21.
[0045] In order to avoid cracking in the carrier and in the
epitaxial semiconductor layer, it is therefore necessary to choose
a carrier material whose thermal expansion coefficient a.sub.T does
not differ too greatly either from the thermal expansion
coefficient of the substrate a.sub.S or from the thermal expansion
coefficient of the semiconductor layer a.sub.HL. The radiation
profile and the pulse length of the laser radiation also influence
the choice of a suitable thermal expansion coefficient a.sub.T, as
will be explained in even more detail further below.
[0046] A preferred refinement of the method according to the
invention provides for the thermal expansion coefficient of the
carrier a.sub.T to be chosen to be nearer to the thermal expansion
coefficient of the semiconductor layer a.sub.HL than to the thermal
expansion coefficient a.sub.S of the substrate. Such a choice
enables the formation of cracks in the semiconductor layer to be
effectively reduced or wholly avoided.
[0047] In this case it is expedient if the thermal expansion
coefficient of the carrier a.sub.T differs from the thermal
expansion coefficient a.sub.S of the substrate by 45% or less,
preferably by 40% or less.
[0048] In particular, for a sapphire substrate having a thermal
expansion coefficient of
a(Al.sub.2O.sub.3)=7.5*10.sup.-6K.sup.-1
[0049] a carrier material is preferred whose thermal expansion
coefficient a.sub.T, although it lies below a(Al.sub.2O.sub.3), is
nevertheless greater than 4.125*10.sup.-6K.sup.-1, in particular
greater than 4.5*10.sup.-6K.sup.-1.
[0050] With regard to the thermal properties of the semiconductor
layer, it is advantageous if the thermal expansion coefficient of
the carrier a.sub.T differs from the thermal expansion coefficient
a.sub.HL of the semiconductor layer by 35% or less, preferably by
25% or less. In particular when stripping away a nitride compound
semiconductor layer such as, for example, a GaN-based semiconductor
layer having a thermal expansion coefficient of
a(GaN)=4.3*10.sup.-6K.sup.-1,
[0051] a carrier material is preferred whose thermal expansion
coefficient a.sub.T, although it is greater than a(GaN), is
nonetheless less than 5.8*10.sup.-6K.sup.-1, in particular less
than 5.6*10.sup.-6K.sup.-1.
[0052] Consequently, a carrier having a thermal expansion
coefficient of between 4.125*10.sup.-6K.sup.-1 and
5.8*10.sup.-6K.sup.-1, in particular between 4.5*10.sup.-6K.sup.-1
and 5.6*10.sup.-6K.sup.-1, is particularly well suited to the
stripping-away of a nitride compound semiconductor layer, for
instance a GaN or GaInN layer, from a sapphire substrate.
[0053] Given such a choice of thermal expansion coefficient a.sub.T
a large pulse length of the laser beam pulses, in particular a
pulse length of greater than 15 ns, can be chosen for the
separation of the semiconductor layer from the substrate without
resulting in cracking in the semiconductor layer.
[0054] In a particularly preferred refinement of the invention, the
carrier contains molybdenum. The thermal expansion coefficient of
molybdenum
a(Mo)=5.21*10.sup.-6K.sup.-1
[0055] lies significantly nearer to a(GaN) than, for example, the
thermal expansion coefficient of GaAs where
a(GaAs)=6.4*10.sup.-6K.sup.-1. The abovementioned problem area of
cracking during the laser bombardment is significantly reduced in
the case of the layer assembly molybdenum bonding wafer/GaN
semiconductor layer/sapphire substrate. Moreover, molybdenum is
stable enough such that cracks do not arise during bonding or
during cooling from the bonding temperature to room
temperature.
[0056] In a further preferred refinement of the method according to
the invention, the carrier contains an iron-nickel-cobalt alloy,
which likewise has a favorable thermal expansion coefficient of
a(Fe--Ni--Co)=5.1*10.sup.-6K.sup.-1
[0057] Tungsten having a thermal expansion coefficient of
a(Wo)=4.7*10.sup.-6K.sup.-1
[0058] has also been found to be an advantageous material for the
carrier. It is generally shown that the metallic carrier materials
are scarcely sensitive to cracking on account of their toughness
during the bonding process and during cooling to room
temperature.
[0059] It is also possible within the scope of the invention, in
the selection of the thermal expansion coefficient of the carrier,
to permit a greater tolerance with regard to the thermal expansion
coefficient of the semiconductor layer if shorter laser pulses are
used. Thus, according to the invention, the thermal expansion
coefficient of the carrier a.sub.T may differ from the thermal
expansion coefficient a.sub.HL of the semiconductor layer by 35% or
more if a small pulse length of the laser beam pulses, in
particular a pulse length of less than approximately 15 ns, is
chosen for the separation of the semiconductor layer from the
substrate. This permits, in particular, the use of a GaAs bonding
wafer where a(GaAs)=6.4*10.sup.-6K.sup.-1 with short pulse
duration.
[0060] A preferred further development of the method according to
the invention provides for the abovementioned carriers having an
adapted thermal expansion coefficient to be used in the stripping
method described above with a laser beam having a plateaulike beam
profile. This also includes, in particular, the advantageous
refinements described, such as the use of an excimer laser, for
example with XeF, XeBr, XeCl, KrCl or KrF as laser-active medium,
the formation of a rectangular or trapezoidal spatial radiation
profile, the selection of an emission wavelength of between 200 nm
and 400 nm, the downstream arrangement of suitable optical
arrangements and/or a beam homogenizer or the subsequent
irradiation of the substrate in a plurality of individual regions
of the semiconductor layer.
[0061] Furthermore, as already described, prior to stripping, the
semiconductor layer may be soldered onto the carrier by means of a
gold-tin solder, preferably with a high proportion of gold of 65%
by weight to 85% by weight, or by means of a palladium-indium
solder, it optionally being possible for a metallization containing
gold and/or platinum, for example, also to be applied to that side
of the semiconductor layer which is remote from the substrate.
[0062] It has been found as a further advantage of the method
according to the invention that the use of thermally adapted
carriers also solves the problem of inadequate adhesion between
semiconductor layer and carrier, which has been observed in the
past for example in the case of epitaxial GaN layers in conjunction
with GaAs bonding wafers as the carrier. The control of the strain
balance in the entire layer assembly according to the present
invention also includes the bonding metallization and thus provides
an effective remedy with regard to the aforementioned problem area
of adhesion.
[0063] The invention is suitable in particular for semiconductor
layers containing a nitride compound semiconductor. Nitride
compound semiconductors are for example nitride compounds of
elements of the third and/or fifth main group of the Periodic
Table, such as GaN, AlGaN, InGaN, AlInGaN, InN or AlN. In this
case, the semiconductor layer may also comprise a plurality of
individual layers of different nitride compound semiconductors.
Thus, the semiconductor layer may have for example a conventional
pn junction, a double heterostructure, a single quantum well
structure (SQW structure) or a multiple quantum well structure (MQW
structure). Such structures are known to the person skilled in the
art and are therefore not explained in any greater detail at this
point. Such structures are preferably used in optoelectronic
components such as light emission diodes such as light-emitting
diodes (LEDs) or laser diodes.
[0064] It should be noted that generally in the context of the
invention, in particular for nitride compound semiconductors, a
carrier is suitable which contains gallium arsenide, germanium,
molybdenum, silicon or an alloy, for example based on iron, nickel
and/or cobalt. Carriers based on the abovementioned advantageous
materials molybdenum, tungsten or an iron-nickel-cobalt alloy are
preferably used.
[0065] Examples of a suitable substrate for the epitaxial
fabrication of nitride compound semiconductor layers are silicon,
silicon carbide or aluminum oxide or sapphire substrates, sapphire
substrates advantageously being transmissive to the laser radiation
used for the separation of the semiconductor layer, in particular
in the ultraviolet spectral range. This enables irradiation of the
semiconductor layer through the substrate when stripping away the
semiconductor layer.
[0066] The method according to the invention may advantageously be
employed in the case of thin-film chips typically having a
semiconductor layer with a thickness of less than approximately 50
.mu.m. The thin-film chip may be for example an optoelectronic
chip, in particular a radiation-generating chip such as a
luminescence diode chip, for example.
[0067] Further features, advantages and expediencies of the
invention emerge from the description of the following three
exemplary embodiments of the invention in conjunction with FIGS. 1
to 8, in which:
[0068] FIGS. 1a to 1e show a diagrammatic illustration of a first
exemplary embodiment of a method according to the invention on the
basis of five intermediate steps,
[0069] FIGS. 2a and 2b respectively show a diagrammatic
illustration of two variants of a second exemplary embodiment of a
method according to the invention,
[0070] FIGS. 3a and 3b show a diagrammatic illustration of a beam
profile of the laser beam in the case of the method shown in FIG.
2a,
[0071] FIG. 4 shows a diagrammatic illustration of the resulting
intensity distribution in the case of the method illustrated in
FIG. 2a,
[0072] FIG. 5 shows a diagrammatic illustration of a third
exemplary embodiment of a method according to the invention,
[0073] FIGS. 6a to 6c show a diagrammatic illustration of a
production method with the use of Gaussian intensity
distributions,
[0074] FIG. 7 shows a diagrammatic illustration for elucidating the
arising of cracks in the carrier, and
[0075] FIG. 8 shows a diagrammatic illustration for elucidating the
arising of cracks in the semiconductor layer.
[0076] Identical or identically acting elements are provided with
the same reference symbol in the Figures.
[0077] In the first step of the method illustrated in FIG. 1, FIG.
1a, a semiconductor layer 2 is applied to a substrate 1. This may
be a nitride compound semiconductor layer, for example an InGaN
layer, which is grown epitaxially onto a sapphire substrate. More
widely, the semiconductor layer 2 may also comprise a plurality of
individual layers which may contain for example GaN, AlN, AlGaN,
InGaN, InN or InAlGaN and be grown successively onto the substrate
1.
[0078] In the next step, FIG. 1b, the semiconductor layer 2 is
provided with a contact metallization 3 on the side remote from the
substrate. The contact metallization 3 results in a low contact
resistance between the semiconductor layer 2 and an electrical
connection, for example a connecting wire, that is to be fitted in
a later method step. Moreover, the contact metallization 3 improves
the soldering properties of the semiconductor layer 2.
[0079] The contact metallization 3 may be vapor-deposited or
sputtered on for example in the form of a thin gold- and/or
platinum-containing layer.
[0080] Afterward, a carrier 4 is soldered onto the contact
metallization 3, FIG. 1c. The solder 5 used is preferably a
gold-containing solder, for example a gold-tin solder with a gold
proportion of between 65% by weight and 85% by weight, preferably
75% by weight. Such a soldering connection is distinguished by a
high thermal conductivity and a high stability under alternating
temperature loads.
[0081] The soldering connection may be formed at a joining
temperature of 375.degree. C., a comparatively low joining pressure
of less than 1.0 bar being necessary. This low joining pressure
makes it possible, even in the case of very thin semiconductor
layers, to effect a connection to the carrier 4 without mechanical
damage to the semiconductor layer 2.
[0082] The carrier 4 used may be a GaAs wafer, for example, which
has a similar thermal expansion coefficient to that of
sapphire.
[0083] A carrier 4 in the form of a bonding wafer made of
molybdenum is preferably provided. The thermal expansion
coefficients of the bonding wafer a(Mo)=5.21*10.sup.-6K.sup.-1 and
of the sapphire substrate a(Al.sub.2O.sub.3)=7.5*10.sup.-6K.sup.-1
are relatively close together, so that thermally induced strains in
the semiconductor layer 2 are advantageously kept low. Furthermore,
molybdenum is sufficiently tough, so that cracks do not arise in
the molybdenum bonding wafer during bonding and during cooling from
the bonding temperature to room temperature.
[0084] Instead of a GaAs wafer a Ge wafer may also be used in the
case of the invention. The thermal expansion coefficient of
germanium is similar to that of GaAs, so that differences scarcely
result in this regard. However, a Ge wafer has the advantage over a
GaAs wafer that it can be sawn more easily, in which case, in
particular, no arsenic-containing toxic sawing waste is obtained.
Furthermore, Ge wafers are mechanically stabler. Thus, a sufficient
stability is already achieved with a 200 .mu.m thick Ge wafer for
example, whereas the thickness of a corresponding GaAs wafer is
greater than 600 .mu.m. It is advantageous that it is also not
necessary in this case for the Ge wafer to be thinned by grinding
in a further method step. Finally, Ge wafers are generally
significantly more cost-effective than GaAs wafers.
[0085] Preferably, a gold-containing solder or gold itself is used
as solder in conjunction with a Ge wafer. This achieves a
particularly fixed connection to the semiconductor layer. Use is
made particularly preferably of a gold-vapor-deposited Ge wafer,
which may optionally be provided with an AuSb surface layer.
[0086] In the subsequent step, FIG. 1d, the semiconductor layer 2
is irradiated through the substrate 1 with a laser beam 6 having a
plateaulike beam profile 7. The radiation energy is predominantly
absorbed in the semiconductor layer 2 and brings about a material
decomposition at the interface between the semiconductor layer 2
and the substrate 1, so that the substrate 1 can subsequently be
lifted off.
[0087] What is essential in the case of the invention is that the
beam profile and the coupled-in beam power are dimensioned such
that a high temperature that suffices for material decomposition
arises locally at the interface between the substrate 1 and the
semiconductor layer 2, which temperature falls over the layer
thickness of the semiconductor layer to an extent such that the
connection 5 between the carrier 4 and the semiconductor layer is
not impaired, for example by melting.
[0088] The strong mechanical loads that occur on account of the
material decomposition are advantageously taken up by the solder
layer, so that even semiconductor layers having a thickness of a
few micrometers can be stripped nondestructively from the
substrate.
[0089] The transverse beam profile 7 of the laser beam 6 is
likewise illustrated in FIG. 1d. The beam intensity along the line
A-A is plotted. The beam profile 7 has a central region 17, in
which the intensity is essentially constant. Said central region 17
is adjoined laterally by flank regions 18, in which the intensity
falls steeply. Depending on the type of fall, the beam profile is
like a trapezoid (linear fall) or a rectangle in the case of a very
steep fall.
[0090] An XeF excimer laser is particularly suitable as the
radiation source. On account of the high gain and the typical
resonator geometry of excimer lasers, the spatial beam profile is
plateaulike and therefore particularly suitable for the invention.
Furthermore, the high pulse peak intensity of excimer lasers in a
range of 1 kW to 100 MW and also the emission wavelength in the
ultraviolet spectral range are advantageous in the case of the
invention.
[0091] The laser radiation is focused by means of a suitable
optical arrangement through the substrate onto the semiconductor
layer 2, where it has a typical beam area of approximately 1
mm.times.2 mm or more. The intensity distribution within the beam
area is largely homogeneous, an energy density of between 200
mJ/cm.sup.2 and 800 mJ/cm.sup.2 being achieved. This energy density
in conjunction with a homogeneous intensity distribution enables
the semiconductor layer to be separated from the substrate without
any residues.
[0092] This has been demonstrated experimentally by way of example
using an InGaN layer on a sapphire substrate. Specifically, the
InGaN semiconductor layer was irradiated with a pulsed laser beam
from an XeF excimer laser having a wavelength of 351 nm and a pulse
duration of 25 ns. While the sapphire substrate is transparent to
radiation having this wavelength, it is absorbed to a great extent
in the InGaN semiconductor layer. A thin boundary layer at the
junction with the substrate is heated by the energy input to
temperatures of 800.degree. C. to 1000.degree. C. At this
temperature, the semiconductor material decomposes at the laser
spot to liberate nitrogen and the bond between the semiconductor
layer 14 and the substrate 12 separates.
[0093] As an alternative, a comparable separation without any
residues can be carried out using a KrF excimer laser. At
approximately 248 nm, the emission wavelength lies further in the
ultraviolet spectral range. In this case, even with larger beam
cross sections having a dimensioning of 30 mm.times.10 mm, the
energy density, which correspondingly lies between 150 mJ/cm.sup.2
and 600 mJ/cm.sup.2, preferably between 150 mJ/cm.sup.2 and 450
mJ/cm.sup.2, suffices for separating the semiconductor layer from
the substrate without any residues. Furthermore, XeBr, XeCl and
KrCl excimer lasers having an emission wavelength of approximately
282 nm, 308 nm and 222 nm, respectively, have proved to be suitable
for the invention.
[0094] After irradiation with the laser beam, the substrate 1 can
be lifted off, FIG. 1e, in which case the semiconductor layer 2
remains on the carrier 4 largely without any substrate residues and
can be processed further.
[0095] FIG. 2a shows a second exemplary embodiment of a method
according to the invention. In contrast to the method illustrated
in FIG. 1, in this case individual regions 8 of the semiconductor
layer 2 are successively exposed to the laser beam. The
approximately rectangular individual regions 8 are arranged in
area-filling and slightly overlapping fashion. In this case, the
overlap serves to compensate for the drop in intensity in the edge
regions 18 of the beam profile 7. The individual regions are
furthermore arranged in matrix-like fashion, an offset of the
matrix rows with respect to one another being advantageous with
regard to an intensity distribution that is as homogeneous as
possible. An alternative arrangement of the individual regions 8 is
illustrated diagrammaticly in FIG. 2b.
[0096] The beam profile of the laser beam within the individual
region 8 is illustrated in FIGS. 3a and 3b. In FIG. 3a, the
intensity is plotted along the X axis of the coordinate system 9 of
axes depicted in FIGS. 2a and 2b; FIG. 3b shows the corresponding
intensity profile along the Y axis. Both profiles are plateaulike
and have a central region 17a, 17b adjoined by flanks 18a, 18b with
a steep fall in intensity.
[0097] The intensity distribution resulting from this in the case
of the individual irradiation of the semiconductor layer as shown
in FIG. 2a is illustrated in FIG. 4. The intensity along the line
B-B integrated over the entire irradiation time is plotted. The
result is a largely homogeneous, virtually constant intensity
profile over the entire area of the semiconductor layer 2, which
enables the semiconductor layer 2 to be separated from the
substrate 1 in a manner free of residues.
[0098] By contrast, FIG. 6 illustrates a corresponding method
according to the prior art with regard to the beam profile. The
laser used in this case, for example a frequency-tripled Nd:YAG
laser, has an approximately circular beam area with a Gaussian beam
profile 15.
[0099] A gridlike arrangement--corresponding to FIG. 2a or 2b--of
successively irradiated regions 14 of a semiconductor layer is
shown in FIG. 6a.
[0100] The associated beam profile 15, i.e. the intensity profile
along the X axis and the Y axis of the coordinate system 9 of axes,
is illustrated in FIG. 6b. On account of a rotationally symmetrical
intensity distribution, which also results in the circular beam
area, the intensity profile along the two axes is approximately
identical. The intensity profile corresponds to a Gaussian curve
with maximum intensity at the origin of the coordinate system 9 of
axes.
[0101] In order to attain the decomposition threshold with such a
laser beam, it is generally necessary to focus the beam. In this
case, the decomposition threshold is exceeded in the beam center,
while the energy density is too low for a material decomposition in
the edge regions. An approximately constant intensity distribution,
as illustrated in FIG. 4, cannot be achieved in the case of a
gridlike irradiation of a semiconductor layer in accordance with
FIG. 6a. The intensity variation over the entire beam profile and
in particular the pronounced intensity maximum in the beam center
leads to numerous intensity maxima and minima on the semiconductor
layer.
[0102] An exemplary profile 13 of the intensity along the line
C-C--shown in FIG. 6a--integrated over the entire irradiation time
is illustrated in FIG. 6c. The variation of the intensity profile
13 leads to a nonuniform material decomposition, in which case the
decomposition threshold may be undershot in particular in the
minima of the intensity distribution.
[0103] The semiconductor material is preserved at the locations at
which the energy density necessary for the material decomposition
is not attained. On account of the material decomposition in the
vicinity of these locations, if appropriate with evolution of gas
such as nitrogen, for example, in the case of nitride compound
semiconductors, a high pressure may arise locally and wrench
particles out of the substrate. These particles may adhere to the
locations where the semiconductor material has not decomposed, so
that ultimately substrate residues remain on the stripped
semiconductor layer.
[0104] In order to prevent that, the beam intensity could be
increased further in the case of conventional methods. However,
there would then be the risk of damage to the semiconductor layer
due to overheating at the locations of the intensity maxima.
[0105] FIG. 5 illustrates a third exemplary embodiment of a method
according to the invention. In contrast to the method shown in
FIGS. 1 and 2, the laser beam is in this case imaged onto the
semiconductor layer 2 in such a way that a striplike beam area 19
arises. In this case, the beam area 19 has a longitudinal dimension
a and a transverse dimension b, the longitudinal dimension a being
significantly greater than the transverse dimension b. In the case
of an excimer laser 11, a corresponding beam area may be formed for
example by means of a suitable mask optical arrangement 12. The
longitudinal dimension a is preferably greater than a corresponding
dimension of the semiconductor layer 2, so that the semiconductor
layer 2 is completely irradiated in this direction. In this case,
the fall in intensity in the flank regions 18 of the beam profile
does not affect the separation method, since the flank regions 18
lie outside the semiconductor layer 2.
[0106] The semiconductor layer 2 is moved during irradiation in the
direction of the transverse dimension b so that the entire
semiconductor layer 2 is irradiated uniformly. Given a pulsed laser
with a sufficiently short pulse duration, typically in the
nanoseconds range, this once again results in a progressive
irradiation of striplike individual areas on the semiconductor
layer 2, since the semiconductor layer 2 is essentially moved
further between the laser pulses and the irradiation is effected
instantly relative to this movement.
[0107] It goes without saying that the explanation of the invention
on the basis of the exemplary embodiments is not to be understood
as a restriction of the invention thereto. Rather, individual
aspects of the exemplary embodiments can be combined largely freely
within the scope of the invention.
* * * * *